A strange sky!

A strange sky over the Sands of Forvie. I think that this is an example of a supernumerary rainbow.

According to my good friend Wikipedia, a supernumerary rainbow, also known as a stacker rainbow, is an infrequent phenomenon, consisting of several faint rainbows on the inner side of the primary rainbow. The alternating faint rainbows are caused by interference between rays of light following slightly different paths with slightly varying lengths within the raindrops. Some rays are in phase, reinforcing each other through constructive interference, creating a bright band; others are out of phase by up to half a wavelength, cancelling each other out through destructive interference, and creating a gap. Given the different angles of refraction for rays of different colours, the patterns of interference are slightly different for rays of different colours, so each bright band is differentiated in colour, creating a miniature rainbow.

So, there you have it!